Evolution is often portrayed as a tree, with new species branching off from existing lineages, never again to meet. The truth however is often much messier. In the case of adaptive radiation, in which species diversify rapidly to fill different ecological niches, it can be difficult to resolve relationships, and the phylogeny (i.e. evolutionary tree) may look more like a bush than a tree. This is because lineages may continue to interbreed as new species are established, and/or they may diverge and then re-hybridize, resulting in genetically mixed populations (known as admixture). Even after species diverge, the introduction of genes from one species to another (known as introgression) can occur. All of this results in a network of related species, rather than a simple tree. The extent to which these processes occur and their evolutionary and genomic impacts are not well understood, partially due to the 'tree-like' assumptions of the models that are used to construct phylogenies.